Extend the branch-(de)activate functions to work without a parameter. In

Extend the branch-(de)activate functions to work without a parameter. In
this case, when the cursor is on a branch inset (using the AtPoint mechanism)
the branch to which the branch inset belongs will be (de)activated.

This functionality can now be used through the branch inset context menu

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@28987 a592a061-630c-0410-9148-cb99ea01b6c8